A. Field of the Invention
The present invention relates to a method for producing a diaphragm spring, particularly to a method for providing residual stress to a diaphragm spring for a clutch cover assembly by treating the diaphragm spring to a shot-peening process.
B. Description of the Related Art
A diaphragm spring is a disk like member used in a clutch cover assembly. The diaphragm spring typically includes an annular elastic portion and a plurality of lever portions extending radially inwardly from the annular elastic portion. One side of an outer circumferential portion of the diaphragm spring usually engages and is supported by a clutch cover, and the other side of the diaphragm spring typically engages and biases a pressure plate into engagement with a clutch disk. A releasing mechanism having a bearing typically contacts the radially inward ends of the plurality of levers portions. When such a diaphragm spring is made, a shot-peening is performed to the one-side of a diaphragm spring where a tensile stress is caused to improve its fatigue strength. In the shot-peening process, small metal balls having a particle size of from 0.5 to 1.5 mm are shot at the speed of from 50 to 80 m/second against the surface of a diaphragm spring. The surface layer of the diaphragm spring undergoes a small amount of plastic deformation as a result of the shot-peening process.
When such a shot-peening is performed, the surface layer of the diaphragm spring is microscopically extended by numerous shots. However, the surface layer can not extend because it is restricted from the inside layer. Therefore, a large amount of residual stress is imposed on the surface layer, and the residual stress improves the fatigue strength, resulting in the strength of 100 kgf/mm2.
The conventional shot-peening process is performed to only the surface of a diaphragm spring where a tensile stress is typically the greatest. Therefore, deformation such as warpage can occur at the lever portions.